1. Field of the Invention
The invention relates to a vaporized fuel processing device for an internal combustion engine, in which the fuel vapor produced in a fuel tank is introduced into an intake passage of the internal combustion engine and burned while the engine is in operation.
2. Description of the Related Art
In a vaporized fuel processing device for an internal combustion engine mounted on a vehicle or the like, the fuel vapor produced in a fuel tank is introduced into a canister and temporarily adsorbed by the adsorbing material in the canister. While the engine is in operation, a purge is performed, in which the air in the canister is sucked into the intake passage with the use of the negative pressure in the intake passage and air is introduced into the canister through an atmosphere introduction passage. By performing such a purge, the fuel adsorbed by the adsorbing material is desorbed and introduced into the intake passage along with the air, and the fuel desorbed from the adsorbing material is burned in the internal combustion engine.
In the above vaporized fuel processing device, by performing a purge to desorb the fuel adsorbed by the adsorbing material, the adsorbing capacity of the adsorbing material is recovered to suppress the occurrence of saturation of the adsorbing material. Thus, when the purge passage that allows the canister and the intake passage of the internal combustion engine to communicate with each other is clogged and closed, for example, and it therefore becomes impossible to normally perform the purge, the fuel cannot be desorbed from the adsorbing material and, as a result, the adsorbing material is easily saturated.
When the adsorbing material is saturated, the adsorbing material can no longer adsorb the fuel, so that the fuel vapor introduced into the canister passes through the canister and is discharged into the atmosphere through the atmosphere introduction passage.
Thus, in the vaporized fuel processing device for an internal combustion engine as described above, a purge flow detection process is performed, in which the pressure in the passage connected to the canister is detected to make sure that a flow of gas in the passage occurs as a purge is performed based on the pressure, while the purge is performed. In this way, by performing the purge flow detection process to make sure that a flow of gas in the passage connected to the canister occurs as the purge is performed and by making sure that the purge is being normally performed based on the result of the purge flow detection process, the reliability of the purge is ensured.
A vaporized fuel processing device is also available, in which a stop valve and a relief valve are provided between the canister and the fuel tank to hermetically close the fuel tank. In such a vaporized fuel processing device, in which the fuel tank is hermetically closed, when the stop valve or the relief valve is opened during a purge flow detection, the pressure in the passage connected to the canister varies as the valve is opened, so that it becomes impossible to accurately detect the flow of gas accompanying the purge based on the pressure in the passage.
Thus, in a vaporized fuel processing device for an internal combustion engine described in Japanese Patent Application Publication No. 2005-256624 (JP-A-2005-256624) that includes a relief valve that is opened when the pressure in a fuel tank becomes out of a predetermined first pressure range, it is inhibited to perform the purge flow detection process when the pressure in the fuel tank is out of a second pressure range included in the first pressure range and therefore there is a fear that the relief valve is opened. On the other hand, when the pressure in the fuel tank is within the second pressure range and therefore there is no fear that the relief valve is opened, the purge flow detection process is performed.
With such a configuration, it is possible to reduce the possibility that the relief valve is opened while the purge flow detection process is performed and the variation in pressure caused by the opening of the relief valve makes the result of the purge flow detection process incorrect.
However, when the fuel tank is hermetically closed, the decrease in the amount of fuel in the fuel tank due to the operation of the engine causes the pressure in the fuel tank to decrease because of the lowering of the liquid level. Thus, while the engine is in operation, the pressure in the fuel tank tends to be reduced to a level out of the second pressure range, at which level the relief valve can be opened, and therefore, when a configuration is employed, in which the purge flow detection process is performed when the pressure in the fuel tank is within the second pressure range, the number of times the purge flow detection process is performed becomes small.
Specifically, the number of times the purge flow detection process is performed to make sure that the purge is being normally performed becomes small and it therefore becomes difficult to secure the reliability of the purge, although the purge can be performed.